High-gain and high-temperature applicable phototransistor with multiple mono-crystalline silicon-carbide layers on a silicon substrate

ABSTRACT

The present invention relates to the structure and fabrication process of a high-gain monocrystal Silicon Carbide phototransistor applicable at high temperature. In view of the optical gain and applicable temperature of the conventional n-p-n type Silicon Carbide phototransistor are too low for practical usage, the present invention utilizes a newly developed n-i-p-i-n structure to strengthen the intrinsic properties of the element, in order to enhance optical gain of the phototransistor for being able to operate at high temperature steadily.

BACKGROUND OF THE INVENTION

1. Field of Technology

The present invention relates to techniques of inserting each of the twolow impurity layers (i-layer) into base-collector and base-emitterrespectively for a monocrystal Silicon Carbide (SiC) phototransistor,particularly to the construction and fabrication process of the lowimpurity layer and the phototransistor.

2. Prior Art

As more and more specified electronic devices are requested to operateunder severe conditions, the demand of high-gain solid state sensingelements workable at high temperature subsequently grows up time totime. Unfortunately, most part of the known high-gain photodetectors,such as avalanche photodiodes (APD), phototransistors (PT), etc are madeby using narrow bandgap materials like monocrystal silicon, amorphoussilicon, or group III-V compounds. When temperature is elevated, darkcurrent of the mentioned elements will increase rapidly to thusdeteriorate photosensitivity thereof, so that, the operating temperatureis confined less than 100° C. Pallab Bhattacharya, SemicinductorOpto-electronic devices Englewood Cliffs, N.J.: Prentice Hall, 1944.

The wide bandgap semiconductor materials, such as Silicon Carbide (SiC),diamond, and gallium nitride (GaN), etc have been used to fabricate hightemperature electronic elements lately Morkoc, S. Strite, G. B. Gao, M.E. Lin, B. S-verdlov, and M. Burns, “Largeband-gap SiC, III-V nitride,and II-VI ZnSe-based semiconductor device technologies,” J. Appl. phys.,vol. 76, no. 3, pp. 1363-1398, wherein the relative techniques of SiC isthe most matured and most compatible with silicon ICs in fabricatingprocess. SiC do have several vantage features, such as wide bandgap,high electron mobility, and high thermal conductivity, etc L. Harris,Propertis of Silicon Carbide. London, United Kingdom: INSPEC, theinstitution of Electrical Engineers, 1995. However, for the time being,the developments of high temperature SiC phototransistors are curbed dueto lack of high-gains. This weakness is supposed to be brought aboutfrom a drastic recombination of minority carrier in the base of aconventional n-p-n construction thus lower current gain of a transistorand reduce optical gain accordingly B. Casady and R. W. Johnson, “Statusof Silicon Carbide (SiC) as a Wide-bandgap Seminconductor forHigh-Temperature Applications: A Review, ‘Solid-State Electronics, vol.39, no. 10, pp. 1490-1422, 1996.

Aiming at the above-depicted defects, the present invention is topropose a newly developed construction and fabrication process for ahigh-gain monocrystal SiC phototransistor capable of operating at hightemperature.

SUMMARY OF THE INVENTION

1. Objective of the Invention

The invention is intended to provide a high-gain phototransistorworkable at high temperature. As the fabrication process is compatiblewith that of silicon semiconductor basically, it is advantageous forcutting down production cost and enhancing the relative techniques.

2. Description of Technology

In view of the fact that the present high-gain photodetectors made bysilicon amorphous silicon or materials of group IV-V can only operate attemperature lower than 100° C., while Silicon Carbide (SiC)phototransistors of n-p-n type have a gain too poor to fit practicalapplications, hence, this invention has chosen a newly developedn-i-p-i-n structure for making monocrystal SiC phototransistor. Thischoice is also based on an exemplification made to point out thatamorphous silicon n-i-p-i-n phototransistors have larger optical gainwith lesser noise comparing with that of n-p-n type B.S. Wu, C. Y.Chang, Y. K. Fang, and R. H. Lee, “Amorphous Silicon Phototransistoron aGlass Substrate,” IEEE Trans. On Electron Devices, vol. ED-32, no. 11,pp. 2192-2196, 1985 and C. Y. Chang, J. W. Hong, and Y. K. Fang,“Amorphous Si/SiC phototransistors and avalanche photodiodes,” IEEProceedings-J, vol. 138, no. 3, pp. 226-233, 1991.

The monocrystal SiC phototransistor, grown on a silicon substrate, isbasically formed by a 5-layer structure sequentially comprisingcollector electrode (ITO)/collector (n-type monocrystal SiC)/i₁ lowimpurity layer (i-type monocrystal SiC)/base (p-type monocrystal SiC)/i₂low impurity layer (i-type monocrystal SiC)/emitter (n-type monocrystalSiC) (refer to FIG. 1). In addition, a silicon wafer is employed as theindispensable substrate, and a buffer layer added is to reduce effect oflattice mismatch between silicon and Silicon Carbide for obtainingbetter quality SiC film. It is clear that the present invention has twoextra low impurity layers interfacing base-emitter as well asbase-collector respectively than a conventional n-p-n phototransistor.

The element of the present invention is designed to thoroughly depletethe base and the two low impurity layers under zero bias, and atriangular potential barrier is formed between the collector and emitter(refer to FIG. 2A). When bias V_(CE) (V_(CE)>0) and illumination isapplied simultaneously, photo-induced electrons start moving towardcollector, while holes move in the opposite direction. Some of the holesmay accumulate in the barrier region and neutralize some negative spacecharges to lower down the barrier potential (as shown in FIG. 2b).Consequently, a large optical gain may be thus obtained inasmuch as muchmore electrons can move over a lowered barrier to gather and form arelatively larger current. Because of the difference in conductionmechanism between n-i-p-i-n type and n-p-n phototransistor, though theminority carrier recombination may also occur in the former, the effectto optical gain is rather smaller comparing with result of loweredpotential barrier. Therefore, a far larger optical gain may be obtainedin a SiC n-i-p-i-n phototransistor than that of a SiC n-p-n type.

Furthermore, the lowered quantity of potential barrier in n-i-p-i-n typephototransistor B. S. Wu, C. Y. Chang, Y. K. Fang, and R. H. Lee,“Amorphous Silicon Phototransistoron a Glass Substract,” IEEE Trans. OnElectron Device, vol. ED-32, no. 11, pp. 2192-2196, 1985 may beexpressed as Δφ_(b)=(KT/q)ln(qφ_(L)J_(d)) where φL is photo-carrierflux, J_(d) is dark current.

From above formula, we understand that when temperature goes up, anincrement in J_(d) will inevitably cause a decrement in Δφ_(b), however,a larger T will result in a greater Δφ_(b), and this compensationfunction can reduce effect of temperature fluctuation to Δφ_(b), and soto photocurrent. In other words, the optical gain of n-i-p-i-n typephototransistors is relatively temperature-independent. Consequently, bycombination with high temperature characteristics of SiC, the SiCn-i-p-i-n type phototransistor can obtain a far greater optical gainthan that of SiC n-p-n type.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings disclose an illustrative embodiment of the presentinvention, which serves to exemplify the various advantages and objectshereof, and are as follows:

FIG. 1 is a structure view of n-i-p-i-n type monocrystal SiCphototransistor;

FIG. 2 is an energy band diagram of n-i-p-i-n type monocrystal SiCPhototransistor;

FIG. 2a is an energy band diagram of n-i-p-i-n type monocrystal SiCphototransistor under static equilibrium;

FIG. 2b is an energy band diagram of n-i-p-i-n type monocrystal SiCphototransistor under normal bias and illumination;

FIG. 3 denotes i-v relations under different temperatures of n-i-p-i-ntype monocrystal SiC phototransistor;

FIG. 3a denotes i-v relations under room temperature at 25° C. and hightemperature at 300° C. (photocurrent induced); A wavelength 500 nm,power 10 μW incident light is applied;

FIG. 3b denotes i-v relations under room temperature at 25° C. and hightemperature at 350° C. (dark current existed); No incident lightapplied.

FIG. 4 illustrates relations of temperature to photocurrent/dark currentin n-i-p-i-n and n-p-n monocrystal SiC phototransistors;

FIG. 5 denotes relations between optical gain and temperature;

FIG. 6 compares relations of temperature-optical gain between SiC and Siphototransistors.

Symbol remarks:

11: Collector electrode;

12: Collector;

13: Low impurity layer;

14: Base;

15: Low impurity layer;

16: Emitter;

17: Buffer layer;

18: Silicon substrate;

19: Emitter electrode;

41: Characteristic curve of n-i-p-i-n monocrystal SiC phototransistorunder illumination;

42: Characteristic curve of n-i-p-i-n monocrystal SiC phototransistorwithout illumination;

43: Characteristic curve of n-p-n monocrystal phototransistor underillumination;

44: Characteristic curve of n-p-n monocrystal phototransistor withoutillumination;

51: Curve of n-i-p-i-n type monocrystal phototransistor;

52: Curve of n-p-n type monocrystal phototransistor;

61: Curve of n-i-p-i-n type monocrystal phototransistor;

62: Curve of n-p-n type monocrystal phototransistor;

63: Curve of n-i-p-i-n type silicon phototransistor;

64: Curve of n-p-n type silicon phototransistor.

Å=angstrom

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[Preferred embodiment 1]

Please refer to the detailed description and appended diagramsconcerning a preferred embodiment of this invention to be depicted belowfor further understanding to technique contents and effects:

FIG. 1 denotes the structure of n-i-p-i-n type monocrystalphototransistor comprising layers in sequence as collector electrode(ITO)/collector (n-type monocrystal SiC)/i₁ low impurity layer (i-typemonocrystal SiC)/base (p-type monocrystal SiC)/i₂ low impurity layer(i-type monocrystal SiC)/emitter (n-type monocrystal SiC)/emitterelectrode (Ni). A n-(III)silicon wafer serves as the substrate, and abuffer layer employed to reduce bad effect owing to lattice mismatchbetween silicon and Silicon Carbide for growing better quality SiC film;

FIG. 2 is an energy band diagram of n-i-p-i-n type monocrystal SiCphototransistor;

FIG. 2a is an energy band diagram of n-i-p-i-n type monocrystal SiCphototransistor under static equilibrium;

FIG. 2b is an energy band diagram of n-i-p-i-n type monocrystal SiCphototransistor under normal bias and illumination;

FIG. 3 denotes i-v relations under different temperatures of n-i-p-i-ntype monocrystal SiC phototransistor, it is obvious that thephotocurrent is relatively temperature-independent while dark current isaffected much more;

FIG. 4 illustrates relations of temperature to photocurrent/dark currentin n-i-p-i-n and n-p-n monocrystal SiC phototransistors; test made under10-V bias and an incident light of 10 μW 500 nm, concludes thatphotocurrent of n-i-p-i-n type monocrystal SiC phototransistor suffersthe slightest effect by temperature changes;

FIG. 5 denotes relations between optical gain and temperature, whereinthe optical gain is obtained from formula S. M. Sze, Physics ofSemiconductor, 2^(nd) ed., New York: Wiley, 1981

G=[(I_(C)−I_(D))/q]×[hv/p_(in)]

 =[I_(C)−I_(D)]/(P_(in)×λ)]×[hc/q]

where Ic is photocurrent, I_(D) dark current, Pin power of incidentlight, hv incident photon energy, λ wavelength of incident light. Undersome designated conditions as of 10-V bias 25° C. and an incident lightof 10 μW with 500 nm wavelength, the optical gain of n-i-p-i-n typemonocrystal SiC phototransistor is about 145, while that of n-p-n typeis 8 only. It is worthy of attention, the n-i-p-i-n type monocrystal SiCphototransistor can sustain a high-gain of 106 even at 250° C., whichreveals a latent potential at high temperature operation;

FIG. 6 compares relations of temperature-optical gain between SiC and Siphototransistors, wherein a sharp cliff is found around 100° C. for Siphototransistor while SiC phototransistor maintains a relatively wideplateau until 200° C. This fact confirms that SiC is better than Si as amaterial for fabrication of high temperature phototransistors.

[Preferred embodiment 2]

Please refer to the detailed description and appended diagramsconcerning a preferred embodiment of this invention to be depicted belowfor further understanding of the fabrication process:

[1] The 1st step is disassembled into :1-1 To clean and prepare siliconwafers for serving as substrates; 1-2 To move wafers into growingsystem; 1-3 To evacuate the air till a vacuum of 10⁻⁶ Torr achieved; 1-4To elevate temperature to 900° C.; 1-5 To introduce HCL(10 sccm) andH₂(1.21 pm) into the growing system and keeping pressure at 2.5 Torr for10 minutes in order to remove oxidized layer on the wafers; 1-6 To cooldown chamber to room temperature and repeat 1-3.

[2] The 2nd step is to grow a buffer layer at 2250 Å on the wafer: 2-1To introduce SiH₄(12 sccm) and H₂(1.21 pm) into the growing system andkeep pressure at 2.5 Torr; 2-2 To raise temperature till 1200° C.; 2-3To introduce C₃H₈ into the growing system at flow rate increasing from 0to 10 sccm gradually.

[3] The 3rd step is to grow a layer of n-type monocrystal SiC film(n₂-layer) at thickness 550 Å as the emitter; Growing conditionsincluding 10 sccm C₃H₈, 12 sccm PH₃, 12 sccm SiH₄, 1.21 pm H₂, 2.5 Torr,1200° C.

[4] The 4th step is to grow a layer of P⁻ type monocrystal SiC film(i₂-layer) at thickness 550 Å as a low impurity layer; conditionsincluding 10 sccm C₃H₈, 0.03 sccm B₂H₆, 12 sccm SiH₄, 1.21 pm H₂, 2.5Torr, 1200° C.

[5] The 5th step is to grow a layer of p-type monocrystal SiC film atthickness 150 Å as the base, conditions including 10 sccm C₃H₈, 12 sccmB₂H₆, 12 sccm SiH₄, 1.21 pm H₂, 2.5 Torr, 1200° C.

[6] The 6th step is to grow a layer of p type monocrystal SiC film(i₁-layer) at thickness 5500 Å as a low impurity layer, conditionsincluding 10sccm C₃H₈, 0.03 sccm B₂H₆, 12 sccm SiH₄, 1.21 pm H₂, 2.5Torr, 1200° C.

[7] The 7th step is to grow a layer of n-type monocrystal SiC film atthickness 250 Å as the collector, conditions including 10 sccm C₃H₈, 12sccm B₂H₆, 12 sccm SiH₄, 1.21 pm H₂, 2.5 Torr, 1200° C.

[8] The 8th step is to utilize photo-mask and plasma for engraving toform an emitter connecting section, and setting the element's area.

[9] The 9th step is to electroplate the emitter electrode connectingsection with Nickel, then, an annealing process of 700° C.(-)30secrequired to form the emitter electrode.

[10] The final step is to plate the collector with transparent electrode(ITO) to form the collector electrode.

[Features and merits]

1. This invention can improve the optical gain of n-i-p-i-n typemonocrystal SiC phototransistors to a great extent, far larger than thatof n-p-n type which has a poor optical gain (about 8) at roomtemperature.

2. This invention is applicable under high temperature. An n-i-p-i-ntype monocrystal SiC phototransistor can still keep its optical gainabout 106 at 250° C., and hereby We deeply believe that our invention isprobable a break-through for fabricating the semiconductorphotodetectors. Relatively speaking, most part of the present so-calledhigh-gain photodetectors can only operate at temperature less than 100°C.

3. The fabrication process of the present invention is compatible withthat of Silicon made ICs. This compatibility is advantageous for costconsideration and is instrumental to further developments.

Many changes and modifications in the above-described embodiment of theinvention can, of course, be carried out without departing from thescope thereof. Accordingly, to promote the progress in science and theuseful arts, the invention is disclosed and is intended to be limitedonly by the scope of the appended claims.

What is claimed is:
 1. A high-gain phototransistor with multiplemono-crystalline silicon-carbide (SiC) layers on a silicon substratecomprising: a silicon substrate made from an n-type, (111) orientedsilicon wafer; a 2250-Å buffer layer grown on the substrate; a 550-Å-n-type mono-crystalline SiC layer serving as the emitter layer, grownon the buffer layer; a 550-Å low-impurity p-type mono-crystalline SiClayer, grown on the emitter layer; a 150-Å p-type mono-crystalline SiClayer serving as the base layer, grown on the 550-Å low-impurity p-typemono-crystalline SiC layer; a 5500-Å low-impurity p-typemono-crystalline SiC layer, grown on the base layer; a 250-Å thick,n-type mono-crystalline SiC layer serving as the collector layer, grownon the 5500-Å low-impurity p-type mono-crystalline SiC layer; an IndiumTin Oxide (ITO) layer serving as the collector electrode; formed on thesaid collector layer, nickel metal layer serving as the emitterelectrode, formed on the exposed region of the emitter layer.
 2. Ahigh-gain phototransistor with multiple mono-crystalline SiC layers on asilicon substrate according to claim 1, wherein said buffer layer,emitter layer, 550-Å low-impurity p-type mono-crystalline SiC layer,base layer, 5500-Å low-impurity p-type mono-crystalline SiC layer, andcollector layer are successively grown by aRapid-Thermal-Chemical-Vapor-Deposition (RTCVD) method.
 3. A high-gainphototransistor with multiple mono-crystalline SiC layers on a siliconsubstrate according to claim 1, wherein said buffer layer is grown usinga mixed reaction gas of 12 sccm SiH₄, 1.21 pm H₂ and C₃H₈ with a flowrate increasing gradually from zero to 10 sccm, keeping the growthpressure at 2.5 torr, at 1200° C.
 4. A high-gain phototransistor withmultiple mono-crystalline SiC layers on a silicon substrate according toclaim 1, wherein said emitter layer is grown using a mixed reaction gasof 10 sccm C₃H₈, 12 sccm SiH₄, 12 sccm PH₃, and 1.21 pm H₂, keeping thegrowth pressure at 2.5 torr, at 1200° C.
 5. A high-gain phototransistorwith multiple mono-crystalline SiC layers on a silicon substrateaccording to claim 1, wherein the 550-Å low-impurity p-typemono-crystalline SiC layer is grown using a mixed reaction gas of 10sccm C₃H₈, 12 sccm SiH₄, 0.03 sccm B₂H₆, and 1.21 pm H₂, keeping thegrowth pressure at 2.5 torr, at 1200° C.
 6. A high-gain phototransistorwith multiple mono-crystalline SiC layers on a silicon substrateaccording to claim 1, wherein said base layer is grown using a mixedreaction gas of 10 sccm C₃H₈, 12 sccm SiH₄, 12 sccm B₂H₆, and 1.21 pmH₂, keeping the growth pressure at 2.5 torr, at 1200° C.
 7. A high-gainphototransistor with multiple mono-crystalline SiC layers on a siliconsubstrate according to claim 1, wherein the 5500-Å low-impurity p-typemono-crystalline SiC layer is grown using a mixed reaction gas of 10sccm C₃H₈, 12 sccm SiH₄, 0.03 sccm B₂H₆, and 1.21 pm H₂, keeping thegrowth pressure at 2.5 torr at 1200° C.
 8. A high-gain phototransistorwith multiple mono-crystalline SiC layers on a silicon substrateaccording to claim 1, wherein said collector layer is grown using amixed reaction gas of 10 sccm C₃H₈, 12 sccm SiH₄, 12 sccm PH₃, and 1.21pm H₂, keeping the growth pressure at 2.5 torr, at 1200° C.
 9. Ahigh-gain phototransistor with multiple mono-crystalline SiC layers on asilicon substrate according to claim 1, which has an optical gain ofabout 106 at 250° C.